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Abstract

Plasma etching of polyimide through an aluminum mask in an oxidizing atmosphere enables the entire mask to be converted to oxide and removed by a back door etch process. Alternatively, the thickness of aluminum can be increased so that some metal remains to protect the polyimide during subsequent RF sputter cleaning and to enable direct deposition of second level aluminum metallization. As shown in Fig. 1, an aluminum mask 10 of around 250-400 A thickness is used to define via holes 11 through a polyimide insulation layer 12 to enable contact with a first level aluminum metallization level 13 on the surface of wafer 9. Both aluminum layers are covered with thin layers 14 and 15 of native oxide.

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United States

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English (United States)

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Plasma Etching of Polyimide Through an Aluminum Mask

Plasma etching of polyimide through an aluminum mask in an oxidizing
atmosphere enables the entire mask to be converted to oxide and removed by a
back door etch process. Alternatively, the thickness of aluminum can be
increased so that some metal remains to protect the polyimide during
subsequent RF sputter cleaning and to enable direct deposition of second level
aluminum metallization. As shown in Fig. 1, an aluminum mask 10 of around
250-400 A thickness is used to define via holes 11 through a polyimide insulation
layer 12 to enable contact with a first level aluminum metallization level 13 on the
surface of wafer 9. Both aluminum layers are covered with thin layers 14 and 15
of native oxide. During plasma etching in an oxidizing atmosphere the oxide
thickness in layer 10 is increased until all or most of the mask is converted to
oxide. Both the mask and the native oxide can now be removed together by a
so-called "back door etch" process in an aqueous solution of ammonium
phosphate. As an alternative, where a second level of metallization is required,
the thickness of layer 10 can be increased to 400-700 A . In this case, as shown
in Fig. 2, not all the mask is converted to oxide and aluminum remains on the
surface after the back door etch. This aluminum is used to protect the polyimide
from surface damage during RF sputter clean prior to second level metallization.
The aluminum also maintains the front surfa...